David Gentleman has intruded into our lives far more than we realise over the past 40 years. He's turned the Queen's head; slashed a motorway across the grounds of a stately home; subverted a global oil company's logo to help protesters and dressed scores of Penguins in very tasteful jackets - the latter in pleasingly uniform designs for Shakespeare plays and EM Forster novels, among others. With significant exceptions, many of Gentleman's designs and paintings give the impression of buttressing an orderly society, with a celebratory nod to its worthier institutions, civic and commercial.

He is best known for his postage stamps - 103 of them since he was first commissioned by the Post Office in 1962. To mark those four decades, he has published an album of all facets of his work - prints, paintings and woodcuts, covering everything from illustrations for company annual reports, to his own compilations of visual reporting on cities (London, Paris) and countries (India, Italy).

Though he was to become probably the most prolific designer of new postal issues, Gentleman says he wearied of being known popularly only for this area and of the limitations of theme - which was far greater in the 1960s. Another irritation was having always to allow room for the Queen's portrait on his tiny province.

Tony Benn's enthusiasm for good stamp design when he became Postmaster-General in 1964 led Gentleman to suggest doing away with the royal portrait. Benn was game; Harold Wilson did not want to fall out with the monarchy; but the Queen, when Benn spread the designs all over the palace carpet, initially liked the way Gentleman had substituted a profile in silhouette for the conventional full-face portrait. However, says Gentleman, word soon came that the status quo was to be observed. Some years later, he managed to pen HM within a square centimetre, leaving more room to create miniature sagas of, for instance, the Battle of Hastings, the 1996 World Cup and the Millennium.

It's appropriate that Benn will open the exhibition of Gentleman's work at the Royal College of Art in London that parallels the book, on October 7. But though Gentleman, as active as ever at 72, accepts this is a retrospective season, he's restless about his achievements and is looking for a way to fuse all the elements he uses in his future work.

His studio is at the top of his Victorian family house in a green bow of a Camden crescent. This is the floor-wide factory from which his commissioned work has poured; clinically clean, obsessively in order. But while the far view from the studio windows on one side is of Parliament Hill Fields, from the other he can see a Hogarthian picture of drunks, druggies and dossers in doorways. The buildings, streets and open spaces of London, often on the cusp of change, were the subject of a book of paintings and drawings in 1982.

"I would do it differently now; I would put in a lot more people," he says. "In the past, I was more ready to be nostalgic. Even Charing Cross [his 100-metre mural for the platforms at the London tube station, done in 1979] is a backward-looking treatment of a subject. If I was doing it now, I would like it to be something about today, not 600 years ago."

Yet the mural is his biggest project, humanising the tube as it shows medieval craftsmen going about their work. The scenes were done as woodcuts, a technique he has used often in book illustrations, but which he says he has now tired of. "I gave up engraving after Charing Cross because I thought no future commissions would be for such an interesting purpose. And I was getting fed up with the fiddle of engraving. I'd rather draw than engrave, because it's quicker and more direct."

And yet his house, a treasury for every phase of his work, has a glass-fronted cupboard on a landing where all the woodblocks he has ever made are collected. He takes out a little Shakespeare portrait, made for Penguin books, and says he's still pulled "by the charm of wood engraving; you've got this little block which confines you; you can't go over the edge; that makes you a tight designer".

There's a life-enhancing quality about his designs: if streets and buildings, while not reproduced with photographic accuracy, are instantly recognisable, it's because, he says, he likes to understand the structure of whatever he is drawing, so that his version of it looks plausible.

But some critics have noted that he doesn't go over the edge in the work he does. For a long time, there was nothing controversial. Even his commercial work has been for companies he approved of - at the time, at least. No problems with British Steel (he created the logo of two steel bars folded as easily as a blanket), Penguin or the Post Office.

In the early 1960s, he did some covers for a magazine produced by Esso. "In those days, I would have thought of Esso as a well-heeled client; now I'd think of it as a reckless company," he says. So his latest work featuring the Esso symbol shows total change. Commissioned by Greenpeace, it subverts the famous logo by converting the letters "s" into dollar signs.

Gentleman says there was always an undertow of dissent in his views. It has taken some time to surface. In 1968, when artists (among others) were protesting, he was designing those Shakespeare covers. "I wasn't that involved, despite my left-wing dad [who headed the design department at Shell]. I thought those [protesters] were silly, which I don't think now."

What motivated him to produce polemical images was the government's 1986 decision to allow the US to bomb Libya from bases in Suffolk where, for 25 years, he has had a country retreat. He took a sheaf of his anger to Faber - bitter images of Britain being shafted, and dumped on (literally) by America, which were published under the ironic title A Special Relationship.

Commissions continued to come in, some linked to this newly revealed vein of persuasion, from Greenpeace, and from the National Trust. Previously, he'd drawn courtly gardens for NT's posters; now it wanted a strident image to stop a proposed by-pass slicing through Petworth Park in Sussex. He came up with giant tyre-tracks traversing the landscape.

His eye has increasingly turned to figures. He used to prefer drawing buildings because they kept still while he was at work, unlike people. But the fact that the streets of his London book are relatively empty may have something to do with his natural diffidence. Three trips to India changed that; the resulting book, David Gentleman's India, teems with the life of the multitudes; light clothing reveals bodies crouching, bending, standing, stretching, swaying, sitting. Their physical suppleness registered with him; he translated it into an artistic suppleness that makes him more at ease with people as principal subjects. Now he's thinking of drawing the Dickensian drama on his Camden doorstep.

· Artwork by David Gentleman is published by Ebury Press, price £30. A free exhibition of Gentleman's work is at The Royal College of Art, Kensington Gore, London SW7, from October 7 to 13.